Soo, in short, this isn't an error, but rather, it's our services saying "don't have anything for you" and your system not being able to boot to an OS. I'm assuming there is no pre-built OS on these, right? If there was, you'd probably NOT see this message because it'd be gone so fast.
So the previous post indicated that the system was "known" and that's true. That would indicate that for some reason, BEFORE you re-imaged it, you rebooted it again.
You could delete it from the console AND restart the SBS services...
OR
You could simply assign a boot to PXE task to the computer that is now showing up in the console. The latter is easier.
And the answer is correct about DS 7.5. It will both have more options for responding to PXE, and loose the "computer is remembered" feature of our services that require they be restarted. However, it's not available until later this year, I think Q4, but not sure, so you sorta have to get along without it for a while.